System and method for improving transient stability of grid-connected wind generator system

ABSTRACT

A power electronic control-based capacitor to be used at the terminal of a grid-connected wind generator system for improving the transient stability of the generator following a fault in the network. This eliminates the need of adding auxiliary control devices at the grid side. The wind generator terminal capacitor is controlled through power electronics in such a way as to function both at the steady state and transient conditions maintaining the stability of the wind generator. A power electronic control-based terminal capacitor (“C”) is connected through two back-to-back thyristor switching devices, T1 and T2. The function of the capacitor depends on the triggering or firing-angle of the thyristor switches, which varies from 0 degrees to 180 degrees.

This application claims priority to U.S. Provisional App. No.63/030,801, filed May 27, 2020, which is incorporated herein in itsentirety by specific reference for all purposes.

FIELD OF INVENTION

This invention relates to an apparatus, system, and related methods toimprove the transient stability of a wind generator system following afault in the network.

SUMMARY OF INVENTION

In various exemplary embodiments, the present invention comprises apower electronic control-based capacitor to be used at the terminal of agrid-connected wind generator system for improving the transientstability of the generator following a fault in the network. Theinvention eliminates the need of adding auxiliary control devices at thegrid side. More specifically, no auxiliary devices like a static varcompensator (SVC) or static synchronous compensator (STATCOM) are usedat the grid side during any network fault conditions. The wind generatorterminal capacitor is controlled through power electronics in such a wayas to function both at the steady state and transient conditionsmaintaining the stability of the wind generator. Therefore, the cost ofusing any auxiliary devices can be saved. The present invention isuseful to wind generator manufacturing companies and electric powerindustries.

In several embodiments, a power electronic control-based terminalcapacitor (“C”) is connected through two back-to-back thyristorswitching devices, T1 and T2. The function of the capacitor depends onthe triggering or firing-angle of the thyristor switches, which variesfrom 0 degrees to 180 degrees. At 0 degrees, the full conduction or thefull current flow through the thyristor device, and hence through thecapacitor. At 180 degrees, there is zero conduction or no current flowthrough the capacitor. When the thyristor operates, the capacitorcontrols the reactive power.

It is noteworthy that during a fault period, the firing angle should beclose to zero, so that the reactive power consumption by the capacitoris high. On the other hand, during normal or steady state conditions,the firing angle should be large (e.g., about 160 degrees).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of a wind generator system with a fixed capacitor(“C”).

FIG. 2 shows an exemplary configuration of a STATCOM device.

FIG. 3 shows a thyristor switched single-phase capacitor arrangement.

FIG. 4 shows a control block diagram for firing-angle generation.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In various exemplary embodiments, the present invention comprises apower electronic control-based capacitor to be used at the terminal of agrid-connected wind generator system for improving the transientstability of the generator following a fault in the network.

Due to their low cost, simple and rugged construction, squirrel-cageinduction machines (SCIM) are mostly used for wind generator systems.Since SCIM-based wind generators do not have any physical magneticpoles, in order to produce the required magnetization the wind generator10 always draws reactive power from the power network or grid 20. Butdrawing the reactive power from the power network causes a voltage sagat the generator terminal. Therefore, a capacitor “C” 12 (which isuncontrolled and fixed), as shown in FIG. 1, is connected at theterminal 14 of the wind generator 10 to compensate for the reactivepower demand at the steady state so that the rated terminal voltage andactive power is maintained.

During any faults on the power grid 20, the wind generator 10 terminalvoltage goes very low. Therefore, in order to raise and maintain therated terminal voltage during a fault, typically a reactive powercompensator such as a static var compensator (SVC) or a staticsynchronous compensator (STATCOM) 30, along with the appropriate controltechniques, is used at the grid point. Thus, the transient stability ofthe wind generator system is maintained.

The structure of a typical STATCOM device 30 is shown in FIG. 2. ASTATCOM device typically comprises a step-down transformer 32, a voltagesource converter (VSC) 34 and associated VSC controller, and a smallcapacitor 36. The two reactive power compensators, such as theuncontrolled fixed capacitor 12 and the STATCOM/SVC 30, work for thewind generator under fault conditions to maintain the transientstability of the wind generator 10.

In various exemplary embodiments, the present invention eliminates theneed for a STATCOM/SVC. The transient stability of the wind generator 10thereby is maintained by using only one reactive power compensator 112.Since the uncontrolled fixed capacitor 12 is always needed at theterminal 14 of the wind generator 10, it cannot be removed. In thepresent invention, the auxiliary device (STATCOM/SVC) 30 is removed, andtransient stability is achieved by controlling the fixed capacitor 12through an appropriate technique as described herein.

The invention eliminates the need of adding auxiliary control devices atthe grid side. More specifically, no auxiliary devices like a static varcompensator (SVC) or static synchronous compensator (STATCOM) are usedat the grid side during any network fault conditions. The wind generator10 terminal capacitor 12 is controlled through power electronics in sucha way as to function both at steady state and transient condition, thusmaintaining the stability of the wind generator 10. Therefore, the costof using any auxiliary devices 30 can be eliminated. The presentinvention is useful to wind generator manufacturing companies andelectric power industries.

FIG. 3 shows the power electronic control-based terminal capacitor (“C”)112 connected through the two back-to-back thyristor switching devices,T1 122 and T2 124. This shows one phase of the three-phase system, withthe other two phases having similar configurations. The terminalcapacitor thus requires only six thyristors for a three-phase system.The present invention thus eliminates the need for a step-downtransformer, a voltage source converter with six diodes, a smallcapacitor, and associated VSC controller, that would be required for aprior-art auxiliary device, such as the STATCOM.

The function of the capacitor 112 depends on the triggering orfiring-angle of the thyristor switches 122, 124, which varies from 0degrees to 180 degrees. At 0 degrees, the full conduction or the fullcurrent 130 flow through the thyristor device, and hence through thecapacitor 112. At 180 degrees, there is zero conduction or no currentflow through the capacitor 112. When the thyristor device operates, thecapacitor 112 controls the reactive power, Qc, which can be shownmathematically through equation (1) in FIG. 3.

It is noteworthy that during a fault period, the firing angle should beclose to zero, so that the reactive power consumption by the capacitoris high. On the other hand, during normal or steady state conditions,the firing angle should be large (e.g., about 160 degrees). FIG. 4 showsthe control block diagram to generate the required firing-angle 200 forthe thyristor switches. As seen, Vref 210 is the reference or ratedvoltage, Vmeas 220 is the measured voltage and ΔV 230 is the voltagedifference (the different between Vref and Vmeas) which progressesthrough the non-linear controller 240, and finally the firing-angle isproduced.

Thus, it should be understood that the embodiments and examplesdescribed herein have been chosen and described in order to bestillustrate the principles of the invention and its practicalapplications to thereby enable one of ordinary skill in the art to bestutilize the invention in various embodiments and with variousmodifications as are suited for particular uses contemplated. Eventhough specific embodiments of this invention have been described, theyare not to be taken as exhaustive. There are several variations thatwill be apparent to those skilled in the art.

What is claimed is:
 1. A power generation system, comprising: a wind generator with a terminal configured to connect to a power grid at a grid point; and a reactive power compensator, comprising a power electronic control-based capacitor attached to the terminal.
 2. The system of claim 1, wherein the power electronic control-based capacitor is connected to a first thyristor switch and a second thyristor switch.
 3. The system of claim 2, wherein the first thyristor switch and the second thyristor switch are back-to-back.
 4. The system of claim 2, wherein the amount of current flow through the power electronic control-based capacitor is determined by the firing-angle of the first thyristor switch and the second thyristor switch.
 5. The system of claim 4, wherein the reactive power compensator further comprises a non-linear controller.
 6. The system of claim 5, wherein the non-linear controller produces the firing angle based on a voltage difference between a reference or rated voltage and a measured voltage.
 7. The system of claim 6, wherein the amount of current flow is full at a firing angle of 0 degrees, and the amount of current flow is zero at a firing angle of 180 degrees.
 8. The system of claim 1, wherein the system does not comprise a static synchronous compensator or a static var compensator connected to the grid point.
 9. The system of claim 1, wherein the system does not comprise an uncontrolled fixed capacitor connected to the terminal. 